ARM: ux500: add support for clocksource DBX500 PRCMU

This patch adds support for the DBX500 PRCMU clocksource
to ux500 platforms.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Mattias Wallin <mattias.wallin@stericsson.co>
Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>